fix: Prevent attempt to construct invalid address

When no join was properly registered, a nickname will not be defined.
In that case, attempting to construct the from address for the 'leave'
presence stanza will result in:

java.lang.IllegalArgumentException: The Resourcepart must not be null

This commit prevents that, by verifying that the nickname is non-null,
before sending that stanza.

smack-extensions/src/main/java/org/jivesoftware/smackx/muc/MultiUserChat.java

@@ -756,6 +756,10 @@ public class MultiUserChat {
         // throw.
         userHasLeft();
 
+        if (nickname == null) {
+            return;
+        }
+
         // We leave a room by sending a presence packet where the "to"
         // field is in the form "roomName@service/nickname"
         Presence leavePresence = new Presence(Presence.Type.unavailable);